Calcination of rocks and ores of materials such as metal carbonates to produce their oxide forms is a major contribution to green house gas emissions. Calciners that are used today are generally vertical kilns that are optimised for their overall energy efficiency. A typical vertical kiln calcines limestone by consuming 3.6 GJ per tonne of lime in best practice, compared to the thermodynamic limit of 3.18 GJ per tonne. These vertical kilns have generally replaced the earlier, inefficient, horizontal rotary kilns. Other kilns that have been employed for specific uses include fluid bed reactors and circulating fluid bed reactors.
Carbon Dioxide Emissions
It has been recognised that calcination is a major greenhouse gas generator, responsible for 2.5% of all emissions from human activity. The calcination of carbonates produces carbon dioxide intrinsically, while the combustion process, as generally used to provide the heat, also produces carbon dioxide. The mass and energy balances of the two processes demonstrate that each tonne of limestone produces 0.44 tonnes of carbon dioxide, and the combustion of, say, LPG to produce 3.18 GJ of heat produces another 0.21 tonnes of carbon dioxide, giving total emissions of 0.65 tonnes of carbon dioxide per tonne of lime produced under ideal conditions. Non-ideal conditions can produce significantly more carbon dioxide.
In the calcination processes widely used today for both the production of lime and clinker, the carbon dioxide from the intrinsic reaction and the burning of fuel is produced in the calciner, and is vented to the atmosphere as a pollutant. Mediation of this emission is made difficult by the fact that the carbon dioxide is also mixed with nitrogen and other flue gases, so that an expensive step of separating these gases is required as the first step in carbon capture. Industry is currently under considerable pressure to minimise emissions of carbon dioxide, and the separation of carbon dioxide from nitrogen and most of the other gases is currently uneconomical.
Reactors Using Heat Transfer
There have been a number of alternative processes suggested that could be used as the basis for an improved, environmentally beneficial, calcination process. One approach is to separate the combustion and calcination processes by using heat transfer, thereby removing the need to separate the carbon dioxide released from the calcination reaction from the flue gases. Saddy et al (WO 97/01615) describe a thermal radiation furnace in which an external source is used to heat the contents of the calciner by radiative heat transfer. In their system the material is fed through the calciner as a pile controlled by a rotary valve, for example, at the lowest part of the furnace, with a long residence time. This approach has an additional beneficial effect of reducing the concentration of carbon dioxide in the calciner so that the quenching of the reaction by carbon dioxide may be partially suppressed. This quenching is a well studied problem, for example, as reported by E. Cremer, Z. Electrochem, 66 pp. 697-702 (1962). A lowering of the carbon dioxide concentration allows the calcination process to proceed at a lower temperature.
The use of external heating, however, potentially comes at a cost of energy. In conventional kilns that use internal combustion, there is efficient heat transfer to the material, and the heat loss from the flue gases is minimised by careful recuperation of heat back to the feedstock and fuel. Best practice is typically a 20% heat loss. The cost of fuel is generally a large component of the operating cost of a kiln. If burners are used to provide external heat, the heat loss can be considerable, in the order of 30%. Shah et al (U.S. Pat. No. 7,025,940) disclose an approach to external heating, called Flameless Distributed Combustion (FDC), in which the combustion of the fuel occurs as essentially a homogeneous chemical reaction. The requirements are that the fuel, such as natural gas, ethanol, diesel or biodiesel is mixed with air which has been heated such that the temperature in the heater section is above the auto-ignition temperature. To achieve a uniform temperature, the rate of combustion must be slower than the mixing time of the gases in the reactor, and multiple injection points are used. Carbon formation by pyrolysis in the fuel heating section can be suppressed by the injection of CO2 and steam in that section. The heat transfer efficiency of FDC is claimed to be as high as 95%. The uniformity ensures that hotspots do not form on the heat exchange surfaces. Thus the use of FDC in an externally heated calciner can potentially be more efficient than current best practice for conventional kilns with internal combustion. The lower temperature of the system, compared to that of a flame, is such that NOx and CO production is very small. FDC can also be accomplished by incorporating a porous material into the burner, so that the feedback of heat that creates the energy efficiency of FDC is accomplished on a micron scale. The benefit of using a porous material is that the radiative heat transfer from the combuster to the reactor surface is optimised.
The Catalytic Effect of Steam
The use of superheated steam in calcination was proposed by Niles (U.S. Pat. No. 1,798,802, issued 1931), who described a process in which the superheated steam reacts with a fuel placed inside the kiln to produce carbon monoxide. Walker (U.S. Pat. No. 2,068,882, issued 1937) proposed to use superheated steam in place of a vacuum for calcination by electrical heat. Vogel (U.S. Pat. No. 2,784,956, issued 1957) improved the process. A characteristic of these processes is that the feedstock size is significantly larger than 100 microns, so that the role of the superheated steam is either as a reactant to oxidise the introduced fuel or to assist the heat transfer in the partial vacuum to the feedstock.
MacIntyre and Stansel, Ind and Eng Chem 45, 1548-1555 (1953), conducted calcination experiments of limestone and dolomite, and demonstrated that the temperature for calcination of limestone under the experimental conditions used decreases from 910° C. in air to 700° C. in superheated steam, and of dolomite from 690° C. in air to 550° C. in superheated steam, thereby suggesting a catalytic effect for a given carbon dioxide partial pressure. In their experiments, the carbon dioxide and steam were pumped from the system at a rate such that the deleterious effect of the back reaction was reduced. Terry and McGurk, Trans Inst Mining and Metallurgy, 103, C62-C68 (1994) conducted Differential Thermal Analysis experiments, and proposed that the catalysis of limestone by superheated steam occurs through an activated calcium bicarbonate intermediate. Thompson et al, Chem Eng Sci, 50, 1373-1382 (1995), using dynamic X-ray diffraction evaluated the kinetics of the catalysis, and demonstrated that the catalysis occurred by the adsorption of water molecules onto the surface, which weakened the binding of carbon dioxide. They showed that the catalytic effect increased with temperature, with the enhancement depending on the superheated steam partial pressure.
In conventional kilns, the feedstock size is significantly greater than 100 microns, and often substantially greater (10-100 mm), such that the catalytic effect of introduced superheated steam is masked by the slower processes of heat and mass transport within the feedstock rocks. In a conventional kiln, superheated steam acting as a reactant for combustion or to assist with heat transfer, would therefore have little impact on the rate of calcination.
Horley (AU 199477474 A1 and AU 2002301717A1) describes a batch calciner which takes advantage of steam catalysis so that calcination of a charge of ground granules can occur during a gravitational drop of a charge through superheated steam. This process is limited in throughput because the chemical energy required for the reaction is provided by the thermal energy of the steam. For the process in Horley to apply to a continuous process, the required feed rate of steam would be excessive.
The Use of Granules in Calciners
Wicke and Wuhrer (U.S. Pat. No. 3,991,172, granted 1976) proposed that the calcination of finely ground limestone (size<100 micron), without superheated steam, with rapid heating and cooling of the order of seconds gave a highly reactive lime (eg as measured by the reaction of the cooled lime with water) because of the high density of chemical defects in the products' lattice structure. Such reactivity is lost if the retention time is too high because the material begins to restructure at the high temperature in a process akin to annealing that removes the chemical defects. Kato and Nakazawa (U.S. Pat. No. 5,653,948, granted 1997) recognised the benefit of producing a fine calcined reactive lime with a size of 1-100 microns, and describe an approach of producing a calcined product with this size in a fluidized bed calciner, which breaks down the feedstock of 100-1500 microns to this size.
Fluid Bed Reactors
Fluid bed reactors generally operate by balancing the gravitational force acting on the granules by the buoyancy of the fluid phase. However, this approach is generally inappropriate for calcining finely ground feedstock of 30-150 microns because the granules are entrained in the gases produced for reasonable gas flows. There are variants of this class of reactor, namely recirculating fluid bed reactors in which the granules are pneumatically circulated through a reactor system using a combination of risers and downers, either of which may be an integral part of the reactor. This approach is used in industrial processes such as catalytic cracking of petroleum. However, such reactors have a long distribution of residence times for the granules because the granules are circulated many times before a fraction of the flow is bled off. This is appropriate for the case in which the granules serve as catalysts, but where the product properties are sensitive to the residence time, for example, where they sinter, recirculation is not desirable.
Flash Calciner
There are flash calciners that are known in the art that use ground granules as the feedstock. These systems can have a lower residence time than conventional kilns. However, in these systems the granules are generally entrained in the combustion gas as a result of using burners within the calciner, so that the output of the calciner is a mixture of the combustion and calcination gases, and are emitted to the atmosphere, as for a standard kiln. In another example of a flash calciner, centrifugal forces from the combustion gasses within the calciner are used to retain the granules in the reactor. These approaches, while perhaps reducing the residence time in the calciner, may still have many of the problems outlined above and result in the same net environmental impact as conventional calcination.
There is prior art that describes the calcination of granules in the form of a pile of powder. Ward and Todd-Davies (GB 2043219) describe a calciner in which a pile of powder is heated by a lance that injects combustion gases into the moving pile of such granules. This reactor is limited by the rate of injection of the combustion gas, which otherwise cause the granules to be entrained in the gas and exhausted with the combustion of gases. Thus the residence time of granules in this calciner design is relatively high. As above, the gas exhausted contains both the combustion gas and the calcination gas, such that the process has the same negative environmental impact as conventional calcination.
A need therefore exists to provide a method and system for calcination of minerals that seeks to address at least one of the above mentioned problems.
At least preferred embodiments of the invention seek to address the requirements for flash calcining a granular material in a reactor system that limits the residence time of the granules and which minimizes environmental impacts